def isPrime(n):
    for i in range (2,int(n**0.5)+1):
        if n%i==0:
            return False
    return True
n=int(input())


n1=n//1000
n2=(n//100)%10
n3=(n//10)%10
n4=n%10


MaxPrime=0

if isPrime(n1) and MaxPrime<n1: MaxPrime=n1
if isPrime(n2) and MaxPrime<n2: MaxPrime=n2
if isPrime(n3) and MaxPrime<n3: MaxPrime=n3
if isPrime(n4) and MaxPrime<n4: MaxPrime=n4



n12=n1*10+n2
if isPrime(n12) and MaxPrime<n12: MaxPrime=n12
n13=n1*10+n3
if isPrime(n13) and MaxPrime<n13: MaxPrime=n13
n14=n1*10+n4
if isPrime(n14) and MaxPrime<n14: MaxPrime=n14


n23=n2*10+n3
if isPrime(n23) and MaxPrime<n23: MaxPrime=n23
n24=n2*10+n4
if isPrime(n24) and MaxPrime<n24: MaxPrime=n24





n34=n3*10+n4
if isPrime(n34) and MaxPrime<n34: MaxPrime=n34



n123=n1*100+n2*10+n3
if isPrime(n123) and MaxPrime<n123: MaxPrime=n123
n124=n1*100+n2*10+n4
if isPrime(n124) and MaxPrime<n124: MaxPrime=n124

n134=n1*100+n3*10+n4
if isPrime(n134) and MaxPrime<n134: MaxPrime=n134
n234=n2*100+n3*10+n4
if isPrime(n234) and MaxPrime<n234: MaxPrime=n234



if isPrime(n) and MaxPrime<n: MaxPrime=n



if n>999 and n<10000:
   print (MaxPrime) 
    
else:   print (0) 
    







